The production of fuel ethanol, or other fuels and chemicals, from lignocellulosic feedstocks provides an attractive alternative to the feedstocks predominantly used to date such as corn starch, sugar cane, and sugar beets. The production of fermentation products from these latter sources cannot increase much further as most of the farmland suitable for the production of these crops is in use. Cellulose is an abundant natural polymer, so there is an enormous untapped potential for its use as a source for fuels and chemicals. Also, lignocellulosic feedstocks to be used for fuel or chemical production are inexpensive as they have limited use. Another advantage of using these feedstocks for fuel or chemical production is that lignin, which is a byproduct of the cellulose conversion process, can be used as a fuel to power the conversion process, thereby avoiding the use of fossil fuels. Several studies have concluded that, when the entire production and consumption cycle is taken into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The conversion of lignocellulosic feedstocks to a fermentation product is usually carried out with a pretreatment process prior to subsequent biological treatments. Pretreatment is usually carried out with addition of a chemical such as an acid or alkali to make the feedstock more amenable to subsequent enzymatic hydrolysis with cellulase enzymes to liberate glucose. The pH of the pretreated feedstock composition resulting from the pretreatment is adjusted to a pH that is suitable for the cellulase enzymes used to produce glucose. The cellulase enzymes then convert the cellulose to glucose and the glucose can then be converted to a fermentation product including ethanol or other fuels or chemicals by yeast or bacterium using known methods. However, one problem with known methods for producing glucose from lignocellulosic feedstocks is that the enzyme requirement is high, which adds significant cost and impedes its commercialization.
Pretreatment of lignocellulosic feedstocks with dilute sulfuric acid at elevated temperature is described in the literature. Pretreatment with sulfuric acid produces an aqueous pretreated feedstock composition in which a large amount of soluble degradation products are present including phenolic lignin and furfural. These compounds inhibit and/or inactivate the cellulase enzymes and thus are most advantageously diluted by adding water, or removed by washing the aqueous pretreated feedstock composition with water prior to carrying out enzymatic hydrolysis. Alternatively, the compounds are removed by treating the liquor by precipitation with lime or other alkali at pH above 8, or by microfiltration methods such as nanofiltration. Unfortunately, there is significant capital and operating cost associated with these processes.
Another chemical pretreatment that has received attention in recent years is pretreatment with sulfur dioxide. Sulfur dioxide is a gas, but when it is dissolved in water, it forms sulfurous acid. Sulfur dioxide and/or sulfurous acid can be added to the lignocellulosic feedstock prior to or during a pretreatment by any of a number of methods, including adding sulfur dioxide gas to the lignocellulosic feedstock or combining dilute sulfurous acid to the lignocellulosic feedstock. Sulfur dioxide and/or sulfurous acid has been reported for use in pretreating dry, presteamed, or prewetted feedstocks such as wood chips. As with sulfuric acid, sulfur dioxide and/or sulfurous acid pretreatment produces soluble compounds that are inhibitory and/or inactivating to cellulase enzymes. These compounds are often diluted with water to decrease the concentration thereof, or removed by washing with water, prior to hydrolysis. The compounds can alternatively be removed by treating the liquor with precipitation using alkali such as lime or by a microfiltration technique, as described previously in connection with sulfuric acid pretreatment. Similar to sulfuric acid pretreatment, there is significant capital and operating cost associated with these processes.